The present invention relates to a method and an apparatus used to apply viscous liquid to one face of plates. More specifically, the present invention relates to a method which is suitably used for applying molten wax uniformly to one face of silicon wafers for use in integrated circuits, prior to a subsequent process such as grinding or abrading the other face of the wafer. The present invention also relates to an apparatus to be used for reducing the above method to embodiments.
When wafers are to be fixed to a carrier plate for mirror-finishing their front faces, generally, molten wax is applied beforehand to the back face of the wafers so as to coat the face with it in a uniform thickness. Then, the wafers are secured to the carrier plate by means of the wax. Subsequently, the front face of the wafer is mirror-finished.
One of the conventional methods to apply molten wax uniformly to a wafer is to spray the molten wax downwardly onto an upwardly facing surface of the wafer which is being delivered by a transport belt or the like (first conventional method). The other method is to drop one or more drops of molten wax onto an upwardly facing surface of the wafer and spin the wafer about its central axis in high speed so that the dropped molten wax is spread uniformly on the surface under centrifugal force (second conventional method).
It is difficult by the first conventional method, however, to apply the molten wax evenly so as to cover the surface in a uniform thickness. Further, the wax applied to the upwardly facing surface might dribble down from the outer peripheral edge of the wafer and stick to the other surface thereof. Still further, particles floating in the air are apt to stick on the waxed surface because the wax is generally sticky. By the second conventional method, on the other hand, wax can be spread evenly by the spin. But, the amount of particles sticking on the waxed surface further increases because a great number of particles are generated and scattered in the air during the spin wherein a considerable portion of the dropped wax is spinned off the wafer and splashes on the surrounding walls. When the wafer is broken accidentally during the spin, scattering fragments may stick on the waxed surface of adjacent wafers. Further, molten wax may drop inadvertently on the wafer after the wax is applied and spread thereover evenly by the spin. Further, bubbles are apt to be contained in the drop of wax which prevent the wax from being spread evenly on the surface and make dimples wherein the wax is not applied to the wafer.
If the wax is unevenly applied, dimples are formed, the wax contains bubbles, or particles are sticked to the waxed surface, the wafer would tilt or deform delicately and irreguraly exerting a defective influence especially in a succeeding photo lithographycal process wherein flatness of the surface must be maintained strictly.